Prior art applicators of the kind within the field of this invention are described in U.S. Pat. No. 5,828,040 and EP-A2-0,746,182 (commonly referred to as PAT in the following). The particular single hybrid mode applicators of this prior art solve a major problem with still earlier prior art: that of uneven heating as evidenced by a patchy and quite unpredictable heating pattern with hot and cold spots (caused by multimode action) and that of excessive edge overheating of loads with high permittivity such as typical compact food items (caused by strong electric horizontal field components which are then parallel to the major edges of the food item).
The particular type of hybrid mode in the applicator described in PAT is characterised by very low vertically (z-) directed impedance, which results in low horizontal (x;y) electric field strengths in relation to those of perpendicularly (z-directed) impinging plane waves. By the choice of a TEy hybrid mode (the feed orientation determines if the mode becomes a TEy or TEx mode), the y-directed electric field component in the applicator becomes zero, which is still more advantageous since edge overheating of y-directed load edges will then not occur.
The particular low impedance applicator mode has preferably its low horizontal index 1 in the direction of transport, since microwave leakage in that direction from the applicators is then minimised. This results in minimum inter-applicator interaction (cross talk) along this direction, and reduces the complexity of the tunnel end microwave choking structures. With the load transport hence in the y direction, the heating pattern of each individual applicator in moving loads becomes striped. This is compensated for by sideways (in the x direction) staggering of following applicators or applicator rows.
The particular low impedance TEy mode has a tendency to create a trapped surface wave mode (a so-called longitudinal section magnetic, LSM, mode) in the region including the undersides of the load items and the metal bottom structure of the tunnel. Even if these modes result in a favourable heating from below in typical food items of about 15 mm or more in height, a problem when several staggered applicators are used is that a significant part of the heating pattern is determined by x-directed standing LSM waves between the sidewalls of the tunnel oven and not only by the fields of the individual applicators.
If the particular TEy mode is used, there may be a tendency of both spreading-out of the applicator fields in the x direction and of inter-applicator crosstalk (i.e. unwanted power transfer between adjacent applicators, either by direct coupling or by LSM mode coupling through the load region). None of the above-mentioned patent documents referred to as PAT do provide any remedy to these imperfections.
In those documents, the preferred embodiments are slot feeds in the top of the applicator sidewalls and the applicator has the TEy11 or TEy21 modes. However, there are cases when larger applicator openings are preferred, in order to achieve a lower power flux density to the load items without a need for reducing the output power of each microwave generator (magnetron). In order for applicators for higher modes, e.g. TEy31 or TEy51 to be successfully designed, other microwave feeding means become necessary.
If the tunnel height is large, there will be an increased likelihood of microwave leakage through the tunnel ends into ambient. For fixed tunnel heights one may then use various kinds of prior art chokes, such as delay lines, quarterwave chokes and chokes which act by mode mismatching. Absorbing media may also be used. Such chokes or absorbers are normally only applied to the horizontal surfaces (top and bottom) of the tunnel opening, but may also be used at the vertical sidewalls in the tunnel opening and choking region. However, if the tunnel height is to be variable, prior art choke structures in the vertical walls become very difficult to employ.